1. Field of the Invention
This invention relates to a spark plug for use for internal combustion engine, and particularly concerns to a spark plug so improved to enable to remove wet and dry carbon deposit by self-cleaning action with a relatively simple structure.
2. Description of Prior Art
In a spark plug for use in internal combustion engine, there is provided a tubular insulator within a metallic shell. Within the insulator, a center electrode is provided so that the center electrode is thermally sealed by a sealant, and supported by a shoulder portion which is provided at an inner wall of the insulator. Then the center electrode is brought its outer surface into tight engagement with an inner surface of the insulator so as to be integrally supported by the insulator. This is to protect the center electrode against an impact which is caused from combustion of air-fuel mixture in a combustion chamber since a front portion of the center electrode is exposed to the combustion chamber.
With the operating passage of the spark plug, a very slight clearance unavoidably occurs between an outer surface of the center electrode and an inner surface of the insulator. The slight clearance catches the air-fuel mixture by capillary action to retain it as droplets of liquid fuel. The droplets of the liquid fuel, thus retained, continuously remain without being replaced by swirl which accompanies the combustion. The liquid fuel absorbs particulate carbon to reduce insulation resistance between the insulator and the center electrode so as to cause misfire.
On the other hand, a deposit of dry carbon placed between the outer surface of the center electrode and the inner surface of the insulator, works to reduce an insulation resistance between the insulator and the center electrode so as to occasion misfire.
Therefore, it is an object of the invention to provide a spark plug which is capable of removing the particulate carbon and the dry carbon deposit to prevent the insulation resistance from unfavorably decreasing, and of contributing to an extended period of service life with a relatively simple structure.